Sunday, November 18, 2012

Zinc…A Double-Edged Metal


Never go to excess, but let moderation be your guide.” (Marcus Tullius Cicero)

 

Moderation and balance is the key to living a healthy lifestyle.  Drinking wine is good for your health, however too much can prove to be fatal.  Medicine should always be taken with care as to not overdose and cause more harm to the patient.  Dr. Jane Flinn of the Department of Psychology at George Mason University has done extensive research on the role of metals in behavior and physiology.  More specifically, her research is focused on the roles of zinc (Zn), copper (Cu), and iron (Fe) in learning, memory, Alzheimer’s, and macular degeneration.
 
Have a cold or the flu?  Take a zinc supplement and your recovery rate will accelerate!  But users beware, too much may actually make you more sick.  Zinc is a mineral that is essential to cellular metabolism and immune functions.  Since the human body does not have a specialized storage system for Zn, supplements are available for daily intake.  A lack of Zn could lead to deficits in taste, hindered growth, anemia, and a weakened immune system.  However, just because too little is bad, too much is not necessarily good.  Dr. Flinn’s research suggests that too much Zn can actually harm learning and memory.  Excessive Zn results in a Cu deficiency, which is associated with a reduction in myelination in the brain.  High amounts of Zn have been found in the hippocampus, amygdala, and prefrontal regions of the brain.  Thus, Zn affects spatial memory, a function of the hippocampus, and fear conditioning, a function of the amygdala and prefrontal cortex.  Although the effects of Zn deficiency have been thoroughly researched on, there are few studies that focus on the effects of elevated Zn in the brain. 



Zinc is essential to maintaining health, but too much zinc can have negative effects.

 
In order to investigate the role of Zn in memory and Alzheimer’s, Dr. Flinn manipulated Zn dosages in rats and evaluated their performance in subsequent memory tasks.  The rats were fed Zn-fortified water that also contained Cu and Fe (10 ppm Zn, 0.25 ppm Cu, and 10 ppm Fe).  Controls were given regular lab water.  The rats were then placed in a Morris Water Maze (MWM) and Stationary Atlantis Platform to test their spatial and reference memory.  Three and nine month latencies were measured to assess the effects of elevated levels of Zn in the rats.  After three months, compared to the controls, rats that were fed with Zn-fortified water were significantly slower when learning how to find the submerged platform in the MWM and Stationary Atlantis Platform.  At nine months, the same Zn-fortified rats were much worse at remembering where the platform was located in the tasks and were more anxious during the task.  The results suggested that increased levels of Zn in the rats’ brains impaired their spatial and reference memory.  

In the lab, mice were also raised to carry an APP mutation so that they developed Alzheimer’s-like plaques.  Dietary enhancements of Zn (10 ppm ZnCO3) were administered to the mice and they were then placed in a MWM.  The Zn-fortified mice that had Alzheimer’s-like plaques showed impaired spatial memory.  The elevated level of Zn along with the onset of Alzheimer’s proved to be detrimental to spatial memory performance.
   
Fear conditioning (FC) was also evaluated in relation to elevated levels of Zn.  There are two parts to fear conditioning:  fear acquisition and fear retention.  New learning must take place to learn that a stimulus is no longer fearful and as a result, the fear becomes extinguished.  The mice were placed in boxes for six minutes and a 20-second tone was given at the end of three, four, and five seconds.  During the last two seconds of the tone, a shock was administered to implement delayed conditioning.  The mice were tested on contextual (same box but no tone) and cued (different box with tone but no shock) fear conditioning extinction.  The results showed the Zn enhanced mice froze more and had a slower fear extinction rate, so the levels of Zn and extinction rate were negatively correlated.


Depiction of FC and extinction.  From left to right: Training, Cued FC extinction, and Contextual FC extinction.


 

In both experiments, which evaluated spatial memory and fear learning, the impairments were alleviated by giving the animals small amounts of Cu.  Post-traumatic stress disorder (PTSD) is characterized by the unnecessary retention of fear, which was modeled by the mice mentioned above.  Individuals with PTSD are unable to learn that certain stimuli are no longer fearful and cannot extinguish their fears.  How can the common basis of fear extinction in Zn-fortified animals and patients with PTSD help develop a new treatment for PTSD?  Possibly in the future, Cu supplements can be tweaked and modified to be used as a treatment for PTSD.  Furthermore, supplementation with copper remediated the effects of increased Zn levels in mice that modeled Alzheimer’s disease.  With these findings, the use of dietary supplements with Zn should be monitored in the elderly population, since they are already at risk for Alzheimer’s.  Increasing Zn consumption with the intention of improving health could backfire and have detrimental effects if the dosage is not controlled.  The research discussed above examined the role of Zn in memory, which can be broadly applied to other illnesses as well, such as dementia, anxiety disorders, depression, and mild cognitive impairment.  However, as the field of medical science continues to advance in drug development, we must evaluate whether these drugs are causing more harm than help.      
   


Sunday, November 4, 2012

α7 nAChR and Gprin 1 Interaction Effects on Neuroregeneration




Neuroregeneration



What would happen to our bodies if cells that are damaged never repair themselves?   Unfortunately, our body structures would eventually break down and no longer function correctly.  Luckily, with vast research conducted, there is a solution!  Neuroregeneration, the regrowth or repair of neural tissues, is imperative to maintaining functions within a system.  By repairing or replacing damaged cells, the damaged area has hopes of gaining back normal functioning.  The regeneration of neural cells has proven to be critical in helping patients with spinal cord injury.   Researchers hope to also find cures to neurodegenerative diseases such as Alzheimer’s and Parkinson’s using neuroregeneration.

Jacob Nordman is a Ph.D. candidate working in the Kabbani Lab of the Krasnow Institute at George Mason University.  He has done extensive research on α7 nAChR and Gprin1 interactions and how they regulate axon and growth cone development, navigation, and regeneration.  He presented his research on α7 nAChR and Gprin1 interactions within hippocampal neurons.



α7 nAChR Along with Gprin 1 Contribute to Axonal Growth



Growth cones, which contain F-actin and microtubules, are situated on axons and dendrites.  They are crucial in guiding axonal development, which allows for the precise wiring of neural circuitry.  To help you picture what a growth cone is, imagine it as an arm and a hand.  There are three layers to a growth cone: central zone (forearm), transitional zone (palm), and peripheral zone (arm used to pull a growth cone to its destination).  Growth cones work with gradients, where a guidance cue can result in either growth or retraction.  The process during which growth cones guide axonal development has seven states:  initiation, formation, guidance, branching, turning, arrest, and retraction.    

The α7 nAChR receptor is a ligand gated calcium channel which has a high affinity for nicotine and ACh, and has been linked to illnesses such as schizophrenia and Alzheimer’s due to its high expression in the hippocampus.  Although there have been studies emphasizing the contribution of α7 nAChR to neuroregeneration, the mechanisms behind this process have not been fully understood.  Gprin1 (G protein regulated inducer of neurite outgrowth 1) is a membrane-bound protein that is highly enriched in growth cones.  Nordman’s research focuses on how α7 nAChR along with Gprin 1 contribute to axonal growth and navigation in hippocampal development.   

Using fluorescent markers to detect proteins and their antibodies within hippocampal brain slices indicated that α7 nAChR and Gprin 1 had the highest expressions during early development at the soma and growth cones, where the cytoskeletal growth demands are highest.  During the early stages of development, neurons are still migrating to their final destinations and are highly dependent on growth cones to guide the paths.  To confirm strong and high overlap of α7 nAChR and Gprin 1 in the soma and growth cones, transfection was used to insert α7 nAChR into Neuro-2a cells, which are neuron-like cells that produce only Gprin 1.  Afterwards, to observe the interactions between the two proteins, immunoprecipitation was used to remove Gprin 1 from the Neuro-2a cells, in order to isolate α7 nAChR.  Upon removing Gprin 1 with siRNA, the direct interaction between α7 nAChR and Gprin 1 weakened, confirming not just the presence of both α7 nAChR and Gprin 1 in the soma and growth cone, but also that there is an active and direct interaction between the two.  

Immunoprecipitation Process


Another experiment on the relationship between α7 nAChR and Gprin 1 was done and it was found the Gprin 1 is a master switch for α7 nAChR.  When there was more Gprin 1, there was an increased growth in axons, more branching, as well as a higer surface area of the growth cones.  Conversely, when there was less Gprin 1, the growth cones retracted and shrinkage was seen.  These results pointed out that whatever is done to Gprin 1 offsets what happens to α7 nAChR.    

The research done by Nordman suggests that α7 nAChR and Gprin 1 together play a critical role in axonal development in the hippocampus.  This study furthers the field of neuroregeneration and can be used to develop drugs to repair neural damage by having more effective ways to regenerate axons and quickly guide them to their destinations.   The overactivation of α7 receptors has been cited in Alzheimer’s disease, causing axons to retract and neural cells to die.  Targeting α7 receptors with antagonists would surely result in useful treatments for Alzheimer’s or other neurodegenerative illnesses.  Myelination is the formation of a layer called the myelin sheath around the axons of neurons.  Myelin increases the speed of impulses through the axon and stabilizes it, which inhibits the regeneration of axons and branching of growth cones.  Thus, inhibiting myelin promotes movement within growth cones.   Inhibiting myelination increases regeneration of axons which is needed to treat neurodegenerative illnesses.  However, can this sort of treatment be a double-edged sword?  This may cause damage to myelin, which contributes to multiple sclerosis.  Neurotrophins are a class of growth factors which contribute to the survival and development of neurons.  Possibly with more studies done on neurotrophins and their interactions with myelin, more safe and effective treatments can be created to treat neurodegenerative illnesses.